Abstract
The silica cell wall of diatoms, a widespread group of unicellular microalgae, is an exquisite example for the ability of organisms to finely sculpt minerals under strict biological control. ...The prevailing paradigm for diatom silicification is that this is invariably an intracellular process, occurring inside specialized silica deposition vesicles that are responsible for silica precipitation and morphogenesis. Here, we study the formation of long silicified extensions that characterize many diatom species. We use cryo-electron tomography to image silica formation in situ, in 3D, and at a nanometer-scale resolution. Remarkably, our data suggest that, contradictory to the ruling paradigm, these intricate structures form outside the cytoplasm. In addition, the formation of these silica extensions is halted at low silicon concentrations that still support the formation of other cell wall elements, further alluding to a different silicification mechanism. The identification of this unconventional strategy expands the suite of mechanisms that diatoms use for silicification.
Many organisms form crystals from transient amorphous precursor phases. In cases where the precursor phases were imaged, they were seen to consist of nanosphere particles. Interestingly, some mature ...biogenic crystals also have a nanosphere particle morphology, but some are characterized by crystallographic faces that are smooth at the nanometer level. There are also biogenic crystals that have both crystallographic faces and nanosphere particle morphology. This highlight presents a working hypothesis, stating that some biomineralization processes involve growth by nanosphere particle accretion, where amorphous nanoparticles are incorporated as such into growing crystals and their morphology is preserved upon crystallization. This process produces biogenic crystals with a nanosphere particle morphology. Other biomineralization processes proceed by ion-by-ion growth, and some cases of biological crystal growth involve both processes. We also identify several biomineralization processes which do not seem to fit this working hypothesis. It is our hope that this highlight will inspire studies that will shed more light on the underlying crystallization mechanisms in biology.
A working hypothesis for the understanding of amorphous-to-crystalline transformations in biogenic skeletal materials formed through transient amorphous precursor phases.
Many organisms form elaborate mineralized structures, constituted of highly organized arrangements of crystals and organic macromolecules. The localization of crystals within these structures is ...presumably determined by the interaction of nucleating macromolecules with the mineral phase. Here we show that, preceding nucleation, a specific interaction between soluble organic molecules and an organic backbone structure directs mineral components to specific sites. This strategy underlies the formation of coccoliths, which are highly ordered arrangements of calcite crystals produced by marine microalgae. On combining the insoluble organic coccolith scaffold with coccolith-associated soluble macromolecules in vitro, we found a massive accretion of calcium ions at the sites where the crystals form in vivo. The in vitro process exhibits profound similarities to the initial stages of coccolith biogenesis in vivo.
Coccoliths are calcitic particles produced inside the cells of unicellular marine algae known as coccolithophores. They are abundant components of sea-floor carbonates, and the stoichiometry of ...calcium to other elements in fossil coccoliths is widely used to infer past environmental conditions. Here we study cryo-preserved cells of the dominant coccolithophore Emiliania huxleyi using state-of-the-art nanoscale imaging and spectroscopy. We identify a compartment, distinct from the coccolith-producing compartment, filled with high concentrations of a disordered form of calcium. Co-localized with calcium are high concentrations of phosphorus and minor concentrations of other cations. The amounts of calcium stored in this reservoir seem to be dynamic and at a certain stage the compartment is in direct contact with the coccolith-producing vesicle, suggesting an active role in coccolith formation. Our findings provide insights into calcium accumulation in this important calcifying organism.
An ugly duckling grows into a swan: Many organisms grow their crystalline mineral phases through the secondary nucleation of nanospheres made of an amorphous precursor phase. Stable amorphous calcium ...carbonate biominerals were used to induce a similar transformation in vitro. The amorphous nanospheres underwent a solid‐phase transformation that resulted in highly ordered calcite crystals composed of aggregated particles (see SEM image).
Many biogenic minerals are composed of aggregated particles at the nanoscale. These minerals usually form through the transformation of amorphous precursors into single crystals inside a privileged ...space controlled by the organism. Here, in vitro experiments aimed at understanding the factors responsible for producing such single crystals with aggregated particle texture are presented. Crystallization is achieved by a two‐step reaction in which amorphous calcium carbonate (ACC) is first precipitated and then transformed into calcite in small volumes of water and in the presence of additives. The additives used are gel‐forming molecules, phosphate ions, and the organic extract from sea urchin embryonic spicules ‐ all are present in various biogenic crystals that grow via the transformation of ACC. Remarkably, this procedure yields faceted single‐crystals of calcite that maintain the nanoparticle texture. The crystals grow predominantly by the accretion of ACC nanoparticles, which subsequently crystallize. Gels and phosphate ions stabilize ACC via a different mechanism than sea urchin spicule macromolecules. It is concluded that the unique nanoparticle texture of biogenic minerals results from formation pathways that may differ from one another, but given the appropriate precursor and micro‐environment, share a common particle accretion mechanism.
Single calcite crystals are grown by amorphous calcium carbonate (ACC) particle‐accretion using a synthetic procedure inspired by biogenic systems. The transformation of solid ACC particles in the presence of certain additives retards the classical dissolution‐precipitation process facilitating growth by a particle‐mediated process. The results provide a mechanistic understanding of biogenic and synthetic single crystal growth.
Cystoliths are amorphous calcium carbonate bodies that form in the leaves of some plant families. Cystoliths are regularly distributed in the epidermis and protrude into the photosynthetic tissue, ...the mesophyll. The photosynthetic pigments generate a steep light gradient in the leaf. Under most illumination regimes the outer mesophyll is light saturated, thus the photosynthetic apparatus is kinetically unable to use the excess light for photochemistry. Here we use micro‐scale modulated fluorometry to demonstrate that light scattered by the cystoliths is distributed from the photosynthetically inefficient upper tissue to the efficient, but light deprived, lower tissue. The results prove that the presence of light scatterers reduces the steep light gradient, thus enabling the leaf to use the incoming light flux more efficiently. MicroCT and electron microscopy confirm that the spatial distribution of the minerals is compatible with their optical function. During the study we encountered large calcium oxalate druses in the same anatomical location as the cystoliths. These druses proved to have similar light scattering functions as the cystoliths. This study shows that certain minerals in the leaves of different plants distribute the light flux more evenly inside the leaf.
Leaf minerals function as internal light scatterers inside leaves. They transfer light from the saturated upper tissue into the light deprived lower tissue. This eases the steep light gradient inside the leaf and improves photosynthetic efficiency on the tissue scale.
Silicate ions increase the thermal stability of the unstable amorphous calcium carbonate (ACC). This effect was observed first by comparing ACC from two different species of cystoliths, small ...calcified bodies formed in the leaves of some plants. The temperature of crystallization to calcite in the silicate-rich cystoliths from M. alba is 100 °C higher than that of the silicate-poor cystoliths from F. microcarpa. The stabilizing effect is confirmed in vitro with synthetic samples differing in their silicate content. With increasing silicate concentration in ACC, the crystallization temperature to calcite also increases. A mechanism of geometric frustration is suggested, whereby the presence of the tetrahedral silicate ion in the flat carbonate lattice prevents organization into crystalline polymorphs.
Diatoms are unicellular algae characterized by silica cell walls. These silica elements are known to be formed intracellularly in membrane-bound silica deposition vesicles and exocytosed after ...completion. How diatoms maintain membrane homeostasis during the exocytosis of these large and rigid silica elements remains unknown. Here we study the membrane dynamics during cell wall formation and exocytosis in two model diatom species, using live-cell confocal microscopy, transmission electron microscopy and cryo-electron tomography. Our results show that during its formation, the mineral phase is in tight association with the silica deposition vesicle membranes, which form a precise mold of the delicate geometrical patterns. We find that during exocytosis, the distal silica deposition vesicle membrane and the plasma membrane gradually detach from the mineral and disintegrate in the extracellular space, without any noticeable endocytic retrieval or extracellular repurposing. We demonstrate that within the cell, the proximal silica deposition vesicle membrane becomes the new barrier between the cell and its environment, and assumes the role of a new plasma membrane. These results provide direct structural observations of diatom silica exocytosis, and point to an extraordinary mechanism in which membrane homeostasis is maintained by discarding, rather than recycling, significant membrane patches.
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